inorganic chemistry · 2013-07-23 · 111. organic receptors for anions iv. introduction to...

PROGRESS IN INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY JOHNS HOPKINS UNIVERSITY BALTIMORE, MARYLAND

VOLUME 46

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS, INC. New York Chichester Weinheim Brisbane Singapore Toronto

Progress in Inorganic Chemistry

Volume 46

Advisory Board

JACQUELINE K. BARTON CALIFORNIA INSTITUTE O F TECHNOLOGY, PASADENA, CALIFORNIA

THEODORE L. BROWN UNIVERSITY O F ILLINOIS, URBANA, ILLINOIS

JAMES P. COLLMAN STANFORD UNIVERSITY. STANFORD. CALIFORNIA

F. ALBERT COTTON TEXAS A & M UNIVERSITY, COLLEGE STATION, TEXAS

ALAN H. COWLEY UNIVERSITY OF TEXAS, AUSTIN. TEXAS

RICHARD H. HOLM HARVARD UNIVERSITY, CAMBRIDGE. MASSACHUSETTS

EIICHI KIMURA HIROSHIMA UNIVERSITY, HIROSHIMA. JAPAN

NATHAN S. LEWIS CALIFORNIA INSTITUTE O F TECHNOLOGY, PASADENA. CALIFORNIA

STEPHEN J. LIPPARD MASSACHUSETTS INSTITUTE O F TECHNOLOGY. CAMBRIDGE, MASSACHUSETTS

NORTHWESTERN UNIVERSITY. EVANSTON. ILLINOIS

EXXON RESEARCH & ENGINEERING C O . , ANNANDALE, NEW JERSEY

KARL WEIGHARDT

TOBIN J . MARKS

EDWARD I . STIEFEL

MAX-PLANCK-INSTITUT. MULHEIM, GERMANY

PROGRESS IN INORGANIC CHEMISTRY

Edited by

KENNETH D. KARLIN

DEPARTMENT OF CHEMISTRY JOHNS HOPKINS UNIVERSITY BALTIMORE, MARYLAND

VOLUME 46

AN INTERSCIENCE@ PUBLICATION JOHN WILEY & SONS, INC. New York Chichester Weinheim Brisbane Singapore Toronto

Cover Illustration of "a molecular ferric wheel" wa?~ adapted from Taft. K . L. and Lippard, S. J . . 1. Anr. Cherri. Soc . , 1990. 1 1 2 , 9629.

Thi\ tcxt is printed on acid-free paper.

A n Interxiencrk Publication

Copynght 5 1997 hy John Wile) & Sons. Inc

Al l rights reserved. Published simultaneously in Canada.

Reproduction or translation of any part of this work beyond that permitted b) Section 107 or 108 of the 1976 United States Copknght Act without the permission of the copyright owner is unlawful. Requests for permission or further informatinn should be addressed to the Permissions Department. John Wile) & Sons. Inc. . 605 Third Avenue. New York. N Y 10158-00 12.

Libra? of Congrebs Catalog Card Number 59- 13035 ISBN 0-47 1 ~ 17992-2

Printed in the United States of Anienca

1 0 9 8 7 6 5 3 3 2 1

Contents

Anion Binding and Recognition by Inorganic Based Receptors PAUL D. BEER AND DAVID K. SMITH

Inorganic Chemistry Laboratory, University of Oxford, Oxford, United Kingdom

1

Copper(I), Lithium and Magnesium Thiolate Complexes: An Overview with Due Mention of Selenolate and Tellurolate Analogues and Related

MAURITS D. JANSSEN, DAVID M. GROVE, and GERARD V A N

KOTEN Department of Metal-Mediated Synthesis, Debye Institute, Utrecht University, Utrecht, The Netherlands

Silver (I) and Gold (I) Species 97

The Role of the Pyrazolate Ligand in Building Polynuclear Transition Metal Systems

GIROLAMO LA MONICA and G. ATTILLO ARDIZZOIA Dipartimento di Chimica Inorganica, Metallorgunica e Analitica and Centro C. N. R . , Universita di Milano, Milano, Italy

Recent Trends in Metal Alkoxide Chemistry RAM C. MEHROTRA and ANIRUDH SINCH

Department of Chemistry, University of Rajasthan, Jaipur, India

Subject Index

Cumulative Index, Volumes 1-46

151

239

455

475

V

Progress in Inorganic Chemistry

Volume 46

Anion Binding and Recognition by Inorganic Based Receptors

PAUL D. BEER and DAVID K. SMITH

Inorganic Chemistry Laboratory, University of Oxford Oxford, UK

CONTENTS

1. INTRODUCTION

11. BIOLOGICAL APPROACHES TO ANION BINDING

A. Binding through Hydrogen Bonding B. Metal Ion Based Anion Binding

111. ORGANIC RECEPTORS FOR ANIONS

IV. INTRODUCTION TO INORGANIC BASED ANION RECEPTORS

V. INORGANIC APPROACHES TO ANION BINDING

A. Neutral Lewis Acidic Receptors 1 , Tin Based Receptors 2. Boron Based Receptors 3 . Silicon and Germanium Based Receptors 4 . Mercury Based Receptors 5. Summary

I . “Robson-Type” Receptors 2. Macrocyclic Receptors 3. Linked Macrocyclic Receptors 4. Macrobicyclic Receptors 5. Receptors Based on a Calixarene Framework 6. Receptors Combining Small Ligating Groups 7. Receptors that Supplement a Single Coordination Interaction 8. Summary

I . Cobaltocenium Based Receptor 2. Metal Ion Cornered Macrocyclic Receptors

B. Multiple Positively Charged Metal Ion Based Coordinative Receptors

C. Charged Receptors Based Primarily on Electrostatic Attraction

Progress in Inorganic Chemistry, Vol. 46, Edited by Kenneth D. Karlin. ISBN 0-471-17992-2 @ 1997 John Wiley & Sons, Inc.

1

2 PAUL D. BEER A N D DAVID K . SMITH

3 . Metalated Calixarene and CTV Receptors 4. Metallacrown Based Receptor 5 . Vanadate Based Receptors 6. Summary

I . Cobaltocenium Based Receptors 2. Fenocene Based Receptors 3 Uranyl Salene Based Receptors 4. Crown Ether Based Receptors 5 . Late Transition Metal Based Receptors 6 . Metalloporphyrin Based Receptor 7 . Ruthenium Based Receptors 8. Summary

D. Recepton Incorporating Hydrogen Bondins

V I . CONCLUSION

REFERENCES

I . INTRODUCTION

In comparison to cation coordination chemistry, anion coordination is a recent development. The birth of this field occurred in the late 1960s with the synthesis of the first artificial host molecules (1-3). It is perhaps surprising that anion recognition was so slow to begin, bearing in mind the importance of anions in many chemical and biological processes. Specific receptors capable of binding anionic guests are dependent on effectively addressing the charac- teristic features of anions, such as their negative charge, their size (larger than analogous cations), their wide variety of shapes, and their pH dependence.

Activity in this field during the 1970s and early 1980s was reported in a range of review articles (4-6). These articles outlined the basic principles involved in anion binding, which helped to delineate the arena for further experimental investigations. In the ensuing years, the investigation of anion recognition systems has continued apace (7-15), and involved the development of a whole new range of receptors, many incorporating metal centers. To date, however, there has not been any comprehensive review of this type of anion receptor based on inorganic systems, an omission this chapter intends to rectify.

Anions are of key importance across many fields of scientific life; making their selective binding and sensing a critical research target, as the following illustrative examples indicate:

Chemically, anions have various roles as catalysts, bases and redox media- tors. The use of receptors to coordinate anions can alter their chemical reativity (4), and may also be helpful for mixture separation or stabilization of unstable species.

ANION RECOGNITION BY INORGANIC BASED RECEPTORS 3

Environmentally, anions pose a considerable pollution problem. In particular, the nitrate anion (used in fertilizers on agricultural land) often pollutes river water to unacceptable levels. This pollution leads to eutrophocation and consequent disruption of aquatic life cycles (16). Radioactive per- technetate anions also cause a pollution problem in the nuclear fuel cycle. Selective binding and sensing of environmentally sensitive anions is, therefore, an important goal.

Biologically, adenosine triphosphate (ATP), the free energy of life processes, is itself an anion, bound by enzymes in order to perform its many metabolic functions. Deoxyribonucleic acid (DNA) is also a polyanion, its binding by proteins being of great importance in transcription and translation processes. Anion-binding biomimicry could therefore yield much information about fundamental biological processes.

Medically, anions are of great importance in many disease pathways. Cystic fibrosis, a genetic illness affecting a significant proportion of society, is caused by misregulation of chloride channels (17). There is, therefore, a real need for selective halide detection, as established methods of chloride analysis are unsuitable for biological applications (18). Cancer is caused by the uncontrolled replication of polyanionic DNA. Anion-binding proteins have also been implicated in the mechanism of Alzheimer’s disease (19).

Many enzymes bind anions very successfully in biological systems; the ma- jority of substrates and cofactors bound by enzymes are anionic (20). It is clearly worth considering the ways in which enzymes carry out this function before going on to discuss synthetic chemical approaches to the problem of anion binding.

11. BIOLOGICAL APPROACHES TO ANION BINDING

A. Binding through Hydrogen Bonding

A recurrent theme in biological chemistry is hydrogen bonding, and the action of receptors for anions is no exception. The crystal structure of the sulfate- binding protein Salmonella typhimurium provides a remarkable example of the coordinative strength of hydrogen bonds for anions (21). The sulfate anion is buried in a cleft 7 A below the surface of the enzyme. The only stabilization for its dinegative charge is provided by neutral hydrogen-bond donors; peptide groups on the protein backbone are particularly important.

More commonly, there is salt bridge hydrogen-bond formation with a protonated amino group (lysine, arginine, or the N-terminus of the protein) which

4 PAUL D. BEER AND DAVID K . SMITH

provides additional anion stabilization through electrostatic attraction (22) . Ex- tensive surveys of the active sites of solely organic, hydrogen-bonding anion- binding proteins have been made (23, 24). Chakrabarti (23) showed that, on average, oxoanions are held by 7 ( + 3 ) hydrogen bonds, of which the protein contributes 5 ( k3) with the rest provided by water molecules.

However. nature is not solely organic as the elegant philosophy of Williams and Frausto da Silva (25) stresses. Frequent use is made of the metal ions available in natural systems to provide additional anion-binding interactions.

B. Metal Ion Based Anion Binding

The guanosine diphosphate (GDP)-bound Ran protein crystal structure (Fig. 1 ) provides a perfect illustration of how a metal ion can be combined with hydrogen-bonding groups to augment the strength of anion binding and form the focus for an anion-binding pocket (26) . (The Ran proteins are located primarily in the nucleus of eukaryotic cells and are involved in protein nuclear import and DNA synthetic control.) The magnesium ion is held in place by four water molecules and two protein residues. This ion binds the anionic GDP substrate in an “end-on” manner. The remaining functionalities of the bound anionic substrate are then satisfied by hydrogen bonding with preorganized sec- tions of the enzyme superstructure.

Lactofenin. an important iron-binding protein (Fig. 2), synergistically binds carbonate anions and iron(II1) (27 ) . The carbonate is bound through a combination of metal-anion coordinate bonds, hydrogen bonds, and electrostatic interactions. It is argued that anion-binding assists iron(II1) binding by causing a buildup o f negative charge at the iron-binding site.

Many structurally refined enzymes have been found to contain more than one metal ion at the active site. These metal ions are often bridged by an anionic substrate.

Superoxide dismutase (an enzyme for destroying biotoxic superoxide anions) contains copper(I1) and zinc(I1) ions at its active site, bridged by an imidazolate anion from a histidine residue (28 ) . Phospholipase C has a number of zinc(l1) ions at its active site. with two of them being bridged by hydroxide and aspartate (from a protein residue) anions (29). Phospholipase C evolved to bind and hy- drolyze negatively charged phosphate esters, illustrating an important principle: the link between anion binding and functioning metal centers. This important concept of function will be returned to at a later stage (Section IV).

Of course. there are many other enzymes that bind anions and many more for which the binding mode is not as yet elucidated. The examples chosen here are merely intended to be illustrative.

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Schematic diagram of the iron- and anion-binding site in human lactofenin. [Reprinted

111. ORGANIC RECEPTORS FOR ANIONS

Dietrich's reviews have provided detailed accounts of early approaches to this type of anion receptor (4, 13). This chapter will, therefore, only briefly outline the basic types of organic receptors for anions as they will prove of relevance in understanding some of the recent inorganic systems that form the main body of discussion.

The first synthetic anion receptor (1) was based on a protonated nitrogen system (1). Compound 1 was shown to bind anions in the cavity in a "katapi- nate" manner (with the hydrogen atoms pointed inward toward the anion) (2). The source of interaction with anions in such a system is therefore twofold: electrostatic attraction and hydrogen bonding. Many protonated polyammoniuni

I


macrocycles were subsequently studied for their ability to bind anions, and in- deed strong interactions were observed in aqueous solution, especially with car- boxylates and phosphates (30-35). Lehn and co-workers (36, 37) in particular, made outstanding contributions in this field of work, synthesizing, for example, compound 2, which was selective for dicarboxylates of specific chain length (as structure (2) illustrated). Compound 3 (BISTREN), when hexaprotonated, se- lectively bound azide anions due to the elliptical nature of the binding site (38, 39).

re7 or 10 m 2 , 3 or 5,6

2

Unfortunately, polyammonium hosts are limited by the pH range over which they are protonated. This pH range is the same as that at which anions (such as phosphate and carboxylate) also begin to protonate. Consequently, the utility of this class of receptor was limited, thus causing the development of guanidinium based hosts.

Guanidinium (4) is protonated across a wider pH range than polyammonium systems and consequently avoids many of their pH limitations. It is extensively

4


used in enzymatic anion-binding systems [such as Staphylococcal nuclease (40, 41)] in the form of arginine residues. It was therefore a natural choice to be incorporated within organic anion receptors. its action relying, once again, on a combination of electrostatic and hydrogen-bonding interactions. Lehn and co- workers (42) were the first to propose the use of polyguanidinium systems as synthetic anion complexones. The strength of anion binding, however, was less than for analogous polyammonium systems. probably due to the greater charge delocalization across guanidinium.

Nonetheless. much excellent use has been made of this system. Receptor 5. for example. extracts p-nitrobenzoate quantitatively from water into chloro- form (43). and the chirality of the receptor allows the possibility of chiral anion recognition (43). Guanidinium has also been incorporated into devices, such as a hydrogen sulfite selective electrode (45). Recently, Mendoza and co-workers (46) reported a chiral double helical array of polyguanidinium strands assem- bled around sulfate templating anions, the first anion centered helical structure.

O Y 0

Q \

0. - 6 \

OYO

/ NO,

5

Another method enabling protonation of nitrogenous host molecules at ac- cessible pH values has been developed by Sessler et al. (47). which was based on the ease of protonation of expanded porphyrins. Studies of sapphyrin [an expanded porphyrin (6)] yielded a crystal structure of diprotonated host with a bound anion, fluoride, found to be in the plane of the macrocyclic ring. These readily protonated expanded porphyrin systems are now well established as receptors for anions. Compound 7. for example, elegantly illustrates two-point binding. with the expanded porphyrin binding the negatively charged phosphate group while the nucleic acid base is complementary to the base of the bound substrate (48).

A different approach to the pH problem involved forming permanently positive nitrogen centers by quaternization. This methodology is exemplified by


Compound 8, which was synthesized by Schmidtchen (49). This receptor provides a fixed-binding site for anions that operates through a combination of electrostatic and hydrophobic forces (50). The absence of donor protons pre- vents any opportunity for hydrogen bonding, but the crystal structure of the iodide complex still indicated that the anion-binding site was in the center of the cavity (51). Zwitterionic hosts such as 9, have also been reported (52, 53). These net neutral hosts prevent the need for the substrate to compete against a


Me

-H3B’

n=6,8 8

+ \

Me

’BH,

n=6,8 9

counteranion in the binding process, hence enhancing the strength of anion binding.

Dipolar electrostatic interactions have also been manipulated for the purposes of anion binding. Macrocyclic receptor 10 was shown to be capable of binding halide anions through interactions with the positive ends of the S = 0 and P = 0 dipoles (54). Evidence was also provided for the simultaneous binding o f primary alkyl ammonium cations (to the oxygen atoms) and halide anions (to the dipoles). This topic of simultaneous cation and anion recognition is of considerable current interest. Further examples will be encountered during this chapter.

Another way of avoiding the problems of pH range and counterion compe- tition is to use a neutral hydrogen-bonding receptor based on amide functionalities. Peptide groups from the protein backbone are, of course, well known to be involved in enzyme anion binding as discussed earlier. Arnide involve-

ANION RECOGNITION BY INORGANIC BASED RECEPTORS 1 1

LO

ment in the binding of anions by synthetic hosts was first suggested by Kimura et al. (32) for the protonated receptor (11). This involvement was later crystallographically proven for azide ion binding (55). In 1986, Pascal et al. (56) prepared 12, the first purely amide based receptor, which had three amide protons pointed into the cyclophane cavity and showed evidence of fluoride binding in dimethyl sulfoxide d, (DMSO-d,) solution. Since this report, considerable use has been made of amides and, in particular, ureas for the construction of neutral anion receptors (57-61).

oso rNH HNJ

0- 0

11 12

All the receptors discussed so far, however, are based on traditional organic chemistry and while often effective, there are several good reasons for ap- proaching the problem of anion binding from a slightly different, inorganic viewpoint.


IV. INTRODUCTION TO INORGANIC BASED ANION RECEPTORS

The organic receptors illustrated above take their inspiration from the first class of biological receptor to be discussed, utilizing solely organic hydrogen bonds. electrostatic attraction. and hydrophobic forces for anion bindnig. Na- ture, however, as shown earlier. casts its net wider in search of effective means of binding anions. thus incorporating metal centers into many of its anion receptors. There are several excellent reasons other than pure biomimicry , however, for attempting to incorporate metal centers:

1 . Source of lilteructions with Anions. Metals are usually either positively charged or formally electron deficient. This knowledge leads to either an enhanced electrostatic interaction with negatively charged substrates or the chance for orbital overlap and formation of bonding interactions, thus increasing the stability of any complex species formed.

2 . Srructurul Factors. Metal compounds often have precisely defined geometries [e.g., Cu(1)-tetrahedral. Cu(I1)-distorted octahedral]. These geometries can be manipulated by the inorganically minded anion recognition chemist to create receptor molecules with well-controlled and interesting relative geometries of ligating groups. This knowledge can be used to enhance selectivity for specifically shaped anions, or create un- usual switching and conformational effects on binding.

3. lr~corporutioti o~j’Fitnctionulity. Metal ions possess a huge range of function. and this is perhaps one of the most compelling reasons for their incorporation into receptor structures. Redox activity. ultraviolet-visible (UV-vis) spectroscopic properties (color), catalytic ability, fluorescent and energy-transfer properties, and radioactivity could all form the basis of potentially useful molecular machines dependent on the recognition of anions. This mechanism could in turn lead to advances in sensor technology (62), anion transport. drug delivery. and catalysis (63); naming only a few applications.

In this chapter, we will try to illustrate the reasons in each case for the incorporation of inorganic centers into the receptors and the advantages conferred to each host by doing so.

In a simplistic treatment, of course, an isolated metal ion could itself be viewed as a receptor for anions as it fulfills the basic criterion of reversible anion binding. The concept of an anion receptor, however, requires a reversal of this metal-centered viewpoint. For the purposes of this chapter, a receptor will be considered as a molecule designed for binding anions, which does so through a combination of bonding interactions (rather than through a single


coordinate bond). The receptors we will discuss are often designed to incorporate anion selectivity and frequently manipulate a variety of noncovalent interactions.

As will be illustrated, the use of inorganic and organometallic chemistry has, in the past 10 years, enabled the development of a rich and exciting range of novel, functional anion receptors. Inorganic anion receptors can be usefully subdivided into four classes dependent on the bonding interactions responsible for anion binding:

1 . Neutral receptors based on multiple Lewis acid-anion orbital overlap in-

2. Positively charged receptors based on multiple coordination interactions

3. Charged receptors based primarily on intermolecular electrostatic attrac-

4. Receptors incorporating hydrogen-bonding interactions.

teractions.

from transition metals.

tion from positively charged metal centers.

V. INORGANIC APPROACHES TO ANION BINDING

A. Neutral, Lewis Acidic Receptors

Lewis acidic centers are, due to their electron deficiency, capable of interacting with anions through an orbital overlap, causing a bonding interaction. Many novel, neutral receptors incorporating multiple numbers of this kind of anion-binding interaction have been developed.

1. Tin Based Receptors

Organotin compounds have been used as neutral carriers for selected anions in membrane electrodes since the late 1960s (64). The compounds generally used, however, were mononuclear tin species such as tnoctyl tin chloride. The mechanism of interaction was elucidated to be the formation of a single-bonding interaction between the four coordinate neutral tin center and the anionic guest (65, 66). As such, these molecules do not fall under our criterion of multiple anion-binding interactions for a designed anion receptor.

The first attempts to marshal a multiple number of Lewis acidic tin centers in order to create a receptor specifically for the purpose of binding anions were made by Newcomb and co-workers (67). In 1984 they reported the synthesis of several tin based macrocycles (e.g., 13 and 14), the first macrocycles to contain Lewis acidic acceptor groups rather than lone-pair donors. Receptor 13 was available in gram quantities making it an ideal receptor for anion-binding in-

14 PAUL D . BEER AND DAVID K . SMITH

/ \ PhzSn\ /SnPh2

(cHz)n n = 4, 5, 6, 8, 10, 12 13

14

vestigations. The first coordination study results were reported in 1987 (68). This kind of host was shown to form both 1 : 1 and 1 : 2 stoichiometric complexes with chloride ions in acetonitrile solution. Stability constants ranged from 400-850 M - ' with little difference between first and second anion coordination (although some uncertainty was expressed over the reliability of these values). This casts some doubt over whether the tin atoms act cooperatively or indepen- dently in this receptor. A small size selective effect was observed (n = 8 binding more strongly than n = 10) and a small macrocyclic effect was observed on comparison with an acyclic analogue.

Macrobicyclic receptors (15) were also reported (69). Binding studies indicated kinetically slower binding than with their macrocyclic analogues (probably due to a more enclosed binding site) The stoichiometry of halide ion binding was exclusively 1 : 1 and encapsulation of the guest anion was postulated (70). Nuclear magnetic resonance (NMR) I "Sn studies were used to illustrate that for receptor 15, with n = 6, the fluoride ion (K = 200 M - I ) was bound five orders of magnitude more strongly than chloride ions ( K 5 0.01 M-') in chlo- roform (71). Crystallography later showed that the fluoride ion was encapsu- lated within the cavity between the two tin atoms (rSnPF = 2.1212.28 A), whereas in the chloride complex, the ion was strongly bound to one tin center but only weakly interacting with the second (72). For fluoride ions, therefore, the enhancement in binding energy is caused by cooperative interactions with both the tin atoms in the host molecule, making 15 a selective fluoride receptor.


Receptor 16 was reported to form a 1 : 1 complex with chloride ions in chlo- roform (K = 500 M - I ) , exhibiting fast exchange on the NMR time scale (73). Such hosts, containing four tin binding sites, were shown to be considerably more effective than mononuclear organotin compounds for chloride binding. In 1991, Newcomb and co-worker (74) published modeling studies for these tin based hosts as well as a crystal structure of 17.

k 4 , X=Ph n=5, X=Ph/CI

16

17

The trinuclear rec ptor 18 and its propyl linked analog 19 have been report d by Jurkschat et al. ( 5, 76). Receptor 18 was shown to transport chloride and bromide ions from water through dichloromethane, but the process was slow. Crystallography of 19a indicated chloride binding between a pair of tin atoms, while NMR studies indicated that 19b involved all three tin atoms in the binding process. No quantitative evaluation of binding affinities was attempted.

16 P A U L D. BEER AND DAVID K . SMITH

18

CI' x X = CI (19a)

X = Me (19a)

Compounds such as 20 have recently been reported and crystallographically proven to form Sn-X-Sn intramolecular bridging interactions (77). These bis- coordinated bromide and iodide ions are observed to be held with bonds of intermediate bonding length, indicative of cooperative tin multicentered anion binding. Multidentate acyclic organyl t in species have recently been used as phosphate selective carriers in polymer based liquid membranes (78). This re- sult indicates the potential for practical application of organic multitin systems in the field of anion recognition.

'R R2 'S"'

I N 2R 'SnO \S"'R'

LO

2. Boron Based Receptors

Although technically a heteroelement rather than a metal, some interesting Lewis acidic receptors for anions have been based on boron-containing systems. The first evidence for this type of anion receptor was published in 1967, one of the earliest examples of anion binding. Compound 21, when compared with 22, exhibited a chelate effect in the binding of methoxide anions (3) .

n F2B BF,

21 22

ANION RECOGNITlON BY INORGANIC BASED RECEPTORS 17

Katz (79, 80) used 23 as a receptor for anions, studying its interaction with hydride, fluoride, and hydroxide ions. Comparison with 24 once again indicated a chelate effect: compound 23 abstracting hydride or fluoride from complexed 24. The crystal structure of the hydride sponge 23, showed the hydride ion bound between the pair of boron atoms with short strong bonds. A crystal structure of the chloride complex was also elucidated and showed the same bridged structure (81).

BMe2 BMe2

b$ 23 24

Compound 25 has been synthesized by Reetz et al. (82,83). It has been used to complex ionic pair species such as KF. The potassium ion is complexed by the crown ether and the fluoride ion is held by a combination of orbital overlap with the Lewis acidic boron atom and electrostatic attraction to the positively charged potassium. This receptor provides an elegant example of the use of a combination of intermolecular forces.

25

A study has also been made of the theoretical propensity of organoboron macrocyclic hosts for anion binding (84). This study indicated that for the hy- pothetical hosts studied, anion inclusion occurred with cavity shrinkage and partial boron rehybridization. This effect would be expected for these anion receptors, which are dependent on orbital overlap interactions for their mode of action.


Recently, Shinkai and co-workers (85) reported a ferroceneboronic acid receptor (26). This receptor showed selective electrochemical recognition of fluoride ions over other halides, the redox potential for the ferrocene unit being perturbed on the addition of F-. There is, however, only one orbital overlap interaction provided by this host and it is likely the selectivity arises solely due to the “hardness” of the fluoride anion.

26

BMe, SiMepR

R = Me,OEt,F 21

Compound 27. a mixed boron-silicon system, was synthesized to investigate the influence of organosilicon on the anion-binding process (86). A fluoride complex was isolated and NMR and crystallographic studies showed that the silicon was involved in binding the anion, but only weakly.

3. Silicon arid Gertnaniutii Based Receptors

In light of the work carried out on boron systems as described above, a silacrown (28) was synthesized in fair yield via nine steps (87). This transported bromide ions more effectively than chloride, but no evidence for the binding mechanism or evaluation of binding affinity was provided. In recent studies, Compound 29 showed, as would be expected, a chelate effect with fluoride ions, exhibiting a high-binding constant (log K > 9 in acetone-d,), which had to be determined via a stepwise procedure (88).

SiF3

SiPh2F

29 Me

28


Germanium based macrocycles (30a-b) have also recently been synthesized (89, 90). Receptor 30b has been shown to transport chloride ions in preference to bromide, although the degree of transport was only 20% in 35L (H20/CH2C12) (91). The expanded hosts (31) have also been synthesized and show similar effects to 30, indicating that ring size is not the only controlling factor in the anion complexation process (92).

R1 = R2= Me 30a

R1=CI, R2=Me 30b

31

4. Mercury Based Receptors

Perhaps the most eye catching of the receptors utilizing Lewis acidic centers are those based on mercury. Mercury is sp (linear) hybridized which has two consequences.

1. Mercury has two empty p orbitals available for orbital overlap with guests and consequently binds anions with practically no energetically unfavor- able geometric reorganization (unlike Sn, Si, B, and Ge, which have to partially rehybridize on anion binding).

20 PAUL D. BEER AND DAVID K. SMITH

2 . Large macrocyclic receptors can be synthesized due to the linear geom- etry (93).

The first literature example of a mercury based receptor (32) for anions was analogous to the chelating boron receptor discussed earlier. The crystal structure indicated that two molecules of Compound 32 asociate with one chloride ion, which sits in a four-coordinate binding site. Solution studies, however, gave results indicative of 1 : 1 binding for halide anions (94, 95). This simple aHgx X=CI,Br

32

HSX

receptor unit was subsequently incorporated into macrocyclic structures (33 and 34) and investigations indicated interactions with electron-donating molecules such as tetrahydrofuran (THF) (96). Unfortunately, there was no marked enhancement in binding attributable to a macrocyclic effect. Receptor 34 has, however, been further functionalized and built into a polymeric membrane electrode that shows selective response to SCN- and CI-, in the presence of NO; and ClO; (97).

33

inorganic chemistry · 2013-07-23 · 111. organic receptors for anions iv. introduction to...

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